docs: def get_child(self, usage_id):
          """Return the child identified by ``usage_id``."""
          if usage_id in self._child_cache:
              return self._child_cache[usage_id]
  
          child_block = self.runtime.get_block(usage_id, for_parent=self)
          self._child_cache[usage_id] = child_block
          return child_block